The preparation of steel strip requires the reduction in gauge of a relatively large and thick steel form by means of passing this form through a reduction mill. During the course of reduction in the mill, the steel is passed between a plurality of driven rollers which are spaced apart a predetermined distance which is less then the thickness of the steel. Each subsequent set of rollers has a thinner gap so that the steel is progressively reduced from the initial thickness to the final desired thickness.
The driven rolls of each stand of the reduction mill not only reduce the thickness of the steel but also pull the steel therebetween. Normally, the stands are set-up in tandem whereby a first stand feeds the strip to the rollers of the next subsequent stand. In this way, the steel may be considered to have been stretched by the cooperative action of the reducing rolls of the tandem pair of stands. This stretching has the undesirable effect, however, of causing the tail end of the strip to be extruded from between the rolls. The extrusion effect is believed, by applicant, to be caused by the pulling of the end of the strip through the rolls rather than it being rolled through.
Applicant believes that the extrusion of the tail end causes small slivers of steel to be formed which break off from the now sharpened tail end of the strip during subsequent rolling in the mill. These steel slivers are usually curved and are capable of scoring or otherwise permanently denting or marking the work rolls. Marking of the work rolls causes the crescent form of the slivers to be repetitively marked onto the coils during the continued rolling operation.
Crescent formation of the finished strip product is undesirable because it is normally a surface defect which requires that the strip either be rerolled to remove it or else be downgraded. Marking of the work rolls by the slivers requires that the rolls be changed or else, naturally, the next subsequent coil will likewise be marked. Those skilled in the art realize that roll changes are expensive, not only because of the cost of the rolls but also because of the downtime required for their changing.
During the rolling operation in the tandem mill, the strip is subjected to high levels of tension because of the cooperative relationship between the rolls of the stands. The tension of the strip between the stands is a function of the differences in the rotational speed between the stands, among other things. Reduction of this tension prior to passage of the tail end through the rolls of a stand has been found to be an effective means of preventing crescent formation. Because of the decreased tension, the tail end is not stretched and therefore not extruded but is, instead, rolled through the stand. The tiny slivers associated with the extrusion of the tail end are therefore avoided and, hence, the work rolls are not gouged.